1. Field of the Invention
The invention relates to a boss fitting which reinforces the structural interface between the preferably filament-wound outer shell of a pressure vessel and a polymeric inner liner of the pressure vessel, and which provides a conduit for transferring fluid to or from the interior of the pressure vessel. A boss comprises a radially extending flange located at the base of an outwardly extending tubular neck. The flange is embedded in the polymeric liner and comprises an upper (axially outer) skirt and a lower (axially inner) skirt. The skirts are spaced to define an annular groove with a textured or knurled surface at the edges of its outer or open end. A plurality of radially spaced apertures extend from the inner or closed end of the groove to a flat under surface of the radially extending flange. The boss is securely integrated with the polymeric liner in a molding procedure. During molding of the polymeric liner, polymeric material flows into the groove and through the apertures, forming a plurality of polymeric anchors whereby the flange is surrounded by and embedded in the polymer on the inside and outside of the boss fitting.
2. Prior Art
It is desirable to make gas storage pressure vessels that are light in weight and yet highly resistant to fragmentation and corrosion damage. To achieve these qualities, pressure vessels can be fabricated from laminated layers such as wound fiberglass filaments or various types of synthetic filaments, bonded together by a thermosetting epoxy resin. An elastomeric or polymeric liner is provided within the filament wound shell to seal the vessel and to prevent fluids in the vessel from contacting and potentially interacting with the composite material.
Filament wound vessels can be constructed in a variety of shapes and typically are cylindrical with a partly spherical end. A boss at the end provides a flowpath to the interior and also structurally joins the internal polymeric liner to the outer composite shell in a way that prevents fluid from penetrating between the liner and the shell. The boss generally comprises a circular flange or support member at the base of a neck that protrudes axially outwardly from the end of the vessel. The support member is attached to the internal liner so as to anchor the boss to the internal liner. A port is defined along the central axis of the neck and the support member. The contents of the pressure vessel communicate with the external environment through the port.
In many applications, composite pressure vessels as described are required to contain fluids at very high pressures. The internal pressure subjects the interface of the boss, the liner and the outer shell to structural loading, which can be extreme. As pressure within the vessel is increased from ambient pressure, bearing stress is generated as the vessel tends to inflate due to the differential pressure between the vessel interior and the ambient pressure. This stress includes forces that operate between the boss and the composite shell. In addition to a stress normal to the plane of the vessel wall (i.e., in a direction that would expel the boss along a line parallel to its axis), shear stress develops between the boss and the internal liner in the plane of the vessel wall, tending to retract the liner radially away from the boss. These stresses also tend to bend the circular flange support member of the boss, outwardly of the vessel toward the center and/or inwardly toward the radial edges.
Sufficient stress can detach the boss from the liner, at least locally. Any such detachment reduces the structural integrity of the vessel, may expose the outer shell or the surfaces between the inner and outer shells to the fluid contents, may contribute to separation of the shells, and may result in leakage from the pressure vessel. It is important to anchor the boss securely to the liner to reduce the possibility of separation.
It has been proposed to include locking structures in a boss for a pressure vessel to better anchor the boss to the liner. For example, U.S. Pat. No. 5,429,845--Newhouse discloses a boss with a support flange having one or more annular grooves for gripping complementary locking tabs formed in the liner. The hub portion or throat is tapered inwardly on its outer surface, providing an inverted inclined bearing surface which engages the outer shell. U.S. Pat. No. 5,476,189--Duvall similarly discloses a boss having a radially projecting support flange with annular grooves. Duvall does not employ a tapered hub but the support flange has annular grooves which mate with tab locks formed in the liner.
Annular locking grooves are helpful to anchor the boss to the liner. However, the respective locking structures, namely the annular grooves and liner tab locks, may not prevent the liner from separating from the boss with sufficient deformation of the vessel in general and the boss in particular. The surfaces of the support flange, which are smooth but for the annular grooves, may permit relative displacement of the inner liner and the support flange under some circumstances, leading to separation.
U.S. Pat. No. 5,518,141--Newhouse discloses another design with annular grooves in the support flange for mating with tab locks in the liner. Newhouse supplements the annular groove locking structure using bolts threaded into the hub of the boss through a support dome disposed inside of the liner. The bolted support dome holds the inner locking tab on the liner captive in its locking groove to resist separation even in the event of deformation of the boss structure. It is not clear how the support dome could be inserted into the vessel and installed from inside the liner to engage over the locking tab and groove, and presumably the liner is molded after the support dome has been attached to the hub.
The foregoing patents are hereby incorporated for their teachings of alternative structures and materials for the polymer lining, the reinforcing shell and the boss.
It would be advantageous to improve the structural connection between the liner of a pressure vessel and a boss having a support flange in a manner that is relatively uncomplicated but produces a robust mechanical attachment of the liner and the boss, and is insensitive to or even improved under conditions in which stresses produce deformation of the boss and its supporting flange, such that relative displacement of the liner and the boss is substantially eliminated.